1. Field of the Invention
The present invention relates to a process for producing sulfonated solid particles, to such sulfonated solid particles, and to compositions containing the sulfonated solid particles. More specifically, it relates to a process for producing sulfonated solid particles, in which a solid particle is brought into contact with gaseous sulfur trioxide, which gaseous sulfur trioxide is obtained by catalytic oxidation of gaseous sulfur dioxide prepared by burning sulfur. It also relates to sulfonated solid particles obtained by the process, and to use thereof.
2. Description of the Related Art
Conventional water-based coloring agents mainly comprise water-soluble dyes. For example, many of ink-jet inks are water-based inks each containing a water-soluble dye as a coloring agent.
However, such dye-based inks are insufficient in light fastness, water resistance, and other fastness properties. Accordingly, instead of water-soluble dyes, use of pigment-based coloring agents such as inks containing pigments that are excellent in light fastness, water resistance, and other physical properties has been investigated. For example, water-based pigment dispersions are increasingly used in inks for use in wiring materials such as felt tip pens, fiber tip pens, and ball point pens. Ink-jet printers using such pigment-based inks have become commercially practical. In addition, such water-based coloring agents containing almost no organic solvents are increasingly used as coloring agents for use in paints and gravure inks, from the viewpoint of environmental protection.
When pigments are used in water-based inks and other water-based coloring agents, they are finely divided in order to improve sharpness, clearness, and transparency of hue of the pigments. Dispersing agents are required to disperse such finely divided pigments into aqueous media homogeneously and to ensure storage stability of the resulting dispersions. However, such dispersing agents alone cannot ensure long-term stability of the pigment dispersions. As a possible solution to improve the long-term stability of the pigment dispersions, sulfo groups or carboxylic acid groups are introduced into the surfaces of the pigments to modify the same.
In general, organic pigments have weak polarity on their surfaces and thereby have insufficient dispersion stability in disperse media, since their surfaces cannot sufficiently adsorb resin dispersing agents in vehicles by action of, for example, hydrogen bonds. To increase affinity between a pigment and a resin dispersing agent and to thereby improve dispersion stability in dispersing media, the pigment is treated with a pigment derivative comprising a pigment and a group with affinity for a resin dispersing agent in a vehicle or a polar group bonded to the pigment. For example, Japanese Patent No. 1241792 proposes a pigment dispersion comprising a pigment, a resin dispersing agent, and a fluidizing agent having an ammonium sulfonate group.
In coloring agents for use in color filters, it is important to finely divide the pigments in order to improve transparency (clarity), sharpness, transmittance, and other physical properties of the resulting color filters and it is particularly important to ensure fluidity of the pigment dispersions, to avoid aggregation of the pigments and to ensure storage stability of the pigments. To these purposes, such pigment derivatives and pigment dispersing agents are used in the coloring agents for use in color filters.
Under these circumstances, attempts have been made to impart polar groups to the pigments. For example, a process has been proposed, in which the surface of a pigment is sulfonated without the use of conventional materials such as sulfuric acid or fuming sulfuric acid to thereby introduce polar groups to the pigment. In this process, a sulfonating agent that is solid at room temperature (e.g., amidosulfonic acid, sulfur trioxide pyridine complex, and sulfur trioxide dimethylformamide complex) is used in the presence of or in the absence of a solvent. This process requires a pretreatment of finely dividing the pigment before sulfonation, in order to sulfonate the pigment surface homogeneously. When the sulfonation is performed in a solvent, the process further requires, after sulfonation, separation of the sulfonated pigment from the solvent, recovery and purification of the solvent, and neutralization of large amounts of the sulfonating agent as a waste and is economically disadvantageous.
When the pigment is sulfonated by mixing with the sulfonating agent that is solid at room temperature and heating the resulting mixture, conditions for sulfonation such as sulfonation temperature and sulfonation time are difficult to control, since the state of the reaction system varies by whether or not the sulfonation temperature is equal to or higher than the melting point of the sulfonating agent, and the sulfonation temperature should be set at a relatively high temperature (e.g., from about 150xc2x0 C. to about 210xc2x0 C.).
Alternatively, carbon powder is sulfonated with gaseous sulfur trioxide obtained by heating and vaporizing solid sulfur trioxide in a laboratory device with a flask. The solid sulfur trioxide used in the process is a highly acidic and highly oxidative compound and is highly toxic to cause inflammation on the mucous membrane and chemical burn on the skin of operators. The solid sulfur trioxide absorbs moisture upon contact with the air and becomes fuming. When it comes into contact with water, it explosively acts upon water and is dissolved therein to yield sulfuric acid. Accordingly, the solid sulfur trioxide in large amounts cannot significantly be used in commercial production from the viewpoints of workability and safety.
Accordingly, it is an object of the present invention to provide a process for commercially producing sulfonated solid particles in large amounts under mild conditions under which the sulfonation is capable of being easily controlled without the use of solvents. After intensive investigations, the present inventors have found that the object can be achieved by sulfonating a solid particles with gaseous sulfur trioxide obtained by catalytic oxidation of gaseous sulfur dioxide, which gaseous sulfur dioxide is prepared by burning sulfur. The present invention has been accomplished based on these findings.
Specifically, the present invention provides, in an aspect, a process for producing a sulfonated solid particle, including the steps of burning sulfur to yield gaseous sulfur dioxide; subjecting the gaseous sulfur dioxide to catalytic oxidation to yield gaseous sulfur trioxide; and sulfonating a dry powdery or granular solid particle with the gaseous sulfur trioxide in a gas phase-solid phase reaction. In another aspect, the present invention provides a sulfonated solid particle produced by the process, a composition and a coloring composition containing the sulfonated solid particle, as well as use thereof.
The term xe2x80x9csolid particlexe2x80x9d as used herein means and includes any solid substance that can be sulfonated on its surface, such as organic pigments, carbon black pigments, conductive carbon black, carbon black for use in batteries, carbon black for use in rubber, water-insoluble dyes, and resinous fine particles. Such carbon black substances may be generically referred to as xe2x80x9ccarbon black fine particlesxe2x80x9d. The solid particle can be of any shape such as powdery, particulate, granular, flaky, and fibril, and these shapes are generically referred to as xe2x80x9cpowdery or granularxe2x80x9d herein.
The process of the present invention can sulfonate solid particles such as organic pigments, carbon black pigments, carbon black and resinous fine particles with the gaseous sulfur trioxide in a gas phase-solid phase reaction and can easily produce sulfonated solid particles with industrial safety at low cost. According to this process, the degree of sulfonation can be freely selected within a broad range. The resulting compositions can be used in known or conventional various applications. In particular, pigment compositions produced according to the process can be used as easily dispersible pigment compositions that are suitable as coloring agents for ink-jet inks and for color filters, as well as coloring agents for regular paints and printing inks. They can also be used as dispersing agents or dispersing assistants for pigments.